3D-Simulation of a Bayard Alpert ionisation gauge using SIMION program

Numerical simulation of a Bayard-Alpert ionisation gauge using SIMION 8.1 software is presented. Gauge sensitivity, being the most important parameter of an ionisation gauge, has been derived using two different approaches. In the first one, SIMION is used to determine mean length of electron trajectories, and ion collection efficiency. From these values, known ionisation cross sections and the gas temperature, the sensitivity can be readily calculated. In the second and more detailed approach, a Monte Carlo method was implemented in the Lua programming language to simulate electron impact ionisation of the gas. The Lua code is incorporated in the SIMION trajectory simulations, in which the ionisation event is modelled by transforming a primary electron into an ion. This approach evaluates the contribution of each trajectory to the gas ionisation. It considers energy dependence of the ionisation probability and the contribution of the electrons backscattered from the grid to the sensitivity. The simulation approaches were tested on the gauge geometry, recently modelled by the CERN group. Very good quantitative agreement with the CERN simulations was obtained using both approaches. Monte Carlo simulations reveal that the relative contribution of electrons backscattered from molybdenum grid to the sensitivity is about 14%.